Electrodeposition of metals



Patented Sept. 5, 1933 v UNITED STATES ELECTRODEPOQITION OF METALS Oliver Balaton and Morris 6. Fowler, cmdale, Aria, assignors to United Verde Copper Company, Clarkdale, Aria, a corporation of Delaware No Drawing.

Application October 17,1930 Serial No.. 489,490

10 Claims. (01. 204-18) This invention relates to electrolysis and has for an object the provision of certain improvements in processes and apparatus for recovering metals from solutions of their salts by electrolysis. The inventionfurther contemplates the provision of improved cathodes or starting sheets for use in electrolytic processes.

In carrying out electrolytic processes for the recovery of metals in accordance with some present practices, electric current is passed through a body or bath of electrolyte or solution of a salt of the metal to be recovered from a suitable anode to a cathode or starting sheet supportedabove the bath and projecting downwardly into the bath. A portion of the cathode or starting sheet extends below the surface of the bath and is normally submerged, and a portion extends above the surface of the bath and is exposed to the atmosphere above the bath. The portion of the cathode 'orstarting sheet at and above the solution line may be'corroded by a spray of electrolyte resulting from gases issuing "from the bath and/or by one or more of the components of the bath. The rate of corrosion is determined largely by the natures of the compounds present in the bath and by the character or type of material employed for the cathode or starting sheet. Corrosion may proceed at such a rate that the cathode or starting sheet will part at the solution j line and the lower portion will drop to the bottom of the electrolytic cell, making contact with the adjacent anode and causing short circuits, and making it diflicult to conduct the process efllciently.

Various procedures have been followed in the different types of electrolytic processes for overcoming or avoiding the harmful effects of cathode or starting sheet corrosion. In processes involving the recovery by electrolysis of metallic zinc from zinc sulphate solutions, the harmful results of corrosion have been eliminated to some extent by employing cathodes or starting-sheets formed of metalsother than those to be recovered during the course of the processes and capable of resisting the corrosive action of the dilute acids of the electrolyte and/or the combined action of the electrolyte and air above the bath at the solution line. In such processes, aluminum cathodes or starting blanks are in almost universal use at the present time.

While electrolytic processes for therecovery of metallic zinc from zinc sulphate solutions may be conducted satisfactorily 'when aluminum cathodes or starting blanks are employed, the

rent efficiency is relatively .low in operations involving the treatment of zinc sulphate solutions containing one or more of the usual impurities when aluminum starting blanks are employed. Our investigations have shown, conclusively, that the low current eiiiciency may be attributed, in

a' large measure, to re-solution of the zinc at points of local concentration of impurities such as lead, copper and cobalt. The presence of copper or cobalt or lead results in the production of small holes extending through-the zinc deposit to the surface of the aluminum. sheet. These holes are of greater diameters at the zinc aluminum interface than at the surface of the zinc deposit and their locations are indicated by black stains on the backs of the sheets of deposited zinc when stripped from the aluminum blank.

Another disadvantage of the use of the aluminum starting blanks results from the fact that .the deposited zinc must be stripped from the aluminum blanks. The stripping operation involves considerable labor and expense and results in the exposure of relatively large surfaces of deposited zinc to oxidizing influences with'the resulting production of relatively large amounts of'dross during the melting operations.

We have discovered that improved results may be obtained in carrying out electrolytic processes by employing cathodes or startingsheets having their exposed portions provided'with protective coatings of certain-materials capable of resisting the corrosive actions of materials of the electrolytes and/or the combined actions of the materials of the electrolytes and the air at the solution lines. We have found that by providing suitable protective coatings we can employ metallic zinc starting sheets in processes involving the recovery of metallic zinc from'zinc sulphate solutions and eliminate many of the dimculties and disadvantageswhich result from using starting sheets or cathode blanks formed of metals other than zinc.

.In carrying out the improved process of our invention we employ a cathode or startingsheet provided with a coating 'or layer of acid resistant material at and adjacent to the. solution line. The coating or layer of protective material may cover the entire exposed portion of the cathode or starting sheet. A coating or layer of protective material may also be applied to the cathode supports. In order that the optimum results may be obtained, we have found it to be advisable to extend the coating r layer of protective material to a point about a half inch or more below the solution lineon the cathode or starting sheet. Apparatus for carrying out the process of the invention will preferably comprise 'a receptacle or container for the electrolyte, and a cathode or starting sheet supported above and extending into the receptacle and having itsupper portion I ness.

provided with a coating of acid resistant material to a point below the normal electrolyte bath level.

Any material which may be applied in thin layers and which is capable of resisting the attack of the dilute acids contained in the electrolyte for the period of deposition may be used for coating the starting sheet orcathode. It is also desirable that the coating material be of such a nature as to permit the passage of metallic ions therethrough when the coating extends below the surface of the electrolyte. In order to avoid the incorporation in the finished cathode of materials which might complicate the recovery of the electrolytic metal in a marketable form, it is advisable to employ, as a coating material, an organic substance which may be readily volatilized or destroyed by heat during the melting operation to which the electrolytical ly deposited metal is normally subjected. For this reason we prefer to use one of the common organic plastic materials such as natural or synthetic rubber products, gums such as those used in preparing varnishes, asphalt products such as mastic, natural resins, and synthetic resins such as phenol condensation products of which the well known bakelite products are examples.

The coating materials may be applied in any suitable manner and in any required amount to provide a protective layer of properv thick- It is only necessary that the layer of protective material adhere closely to the surfaces of the cathodes and be sumciently thick to resist the action of the corrosive influences for the period of deposition. The layer of protective material, or, at any rate, that portion which extends below the solution line should not be so thick as to unduly hinder the passage of positive ions therethrough. We have found that under normal operating conditions a layer of substantially tissue thickness but substantially impervious to the electrolyte is satisfactory. Protective layers of suitable thickness may be formed by applying a solution of the desired protective material to the surface to be coated with a brush or swab or by dipping the cathode or starting sheet in a prepared solution. The solvent employed should be one which evaporates quickly in order that fresh cathodes or starting sheets may be made available quickly. Sheet rubber stretched tightly to lie in contact with and cover an exposed cathode surface provides a suitable protective coating.

While many different materials may be used to provide suitable protective coatings,- we believe that rubber has the greater number of desirable characteristics. Rubber is soluble in some of the more highly volatile commercial solvents. Solutions of any desired consistency or viscosity may be readily obtained and they may be quickly applied to the surfaces to be coated to form films or layers of any desired thickness. A solvent such as benzene or carbon tetrachloride evaporates quickly, permitting the production of a layer of pure rubber within a very short period of time. We have found it to be very satisfactory to em ploy a solution of crepe rubber in benzene for applying a protective layer of rubber to a cathode or starting sheet. Very thin layers of rubber offer satisfactory resistance to the corrosive influences in electrolytic processes. A thin substantially invisible film of rubber on the exposed portion of a cathode or starting sheet will resist the action of corrosive influences for as long a period as three days during the course of an electrolytic zinc process. The portion of the film or coating extending below the solution line does not prevent the passage of positive ions. At the completion of the period of deposition the submerged portion of the rubber coating will be found to be covered with an electrolytic deposit of metal.

Our researches and investigations have clearly demonstrated the practicability of employing metallic zinc cathodes or starting sheets in electrolytic processes for recovering metallic zinc from zinc sulphate solutions when the exposed portions of the'cathode or starting sheet are provided with suitable acid resistant coatings. We propose to modify the heretofore'customary type of electrolytic process for recovering zinc from zinc sulphate solutions by employing a suitably protected metallic zinc cathode or starting sheet instead of the aluminum starting blank in common use. We have found that either electrolytically deposited zinc sheets or rolled zinc sheets may be used satisfactorily. The zincstarting sheets may be supported and connected to the source of supply of electric current in any suitable manner.

According to the preferred electrolytic process of our invention, we cause the zinc to be deposited on submerged surface portions of electrolytically deposited zinc starting sheets. The starting sheets may be formed by electrodeposition of zinc from zinc sulphate solutions on aluminum blanks, electrodeposition continuing for a period of from five to twenty hours, at a current density of from twenty to thirty amperes per square foot. Thin sheets produced in about five hours at a current density of about thirty amperes are rather satisfactory, but they have a tendency to warp, and it is desirable to develop the sheets over a longer period and at a lower current density. Sheets of suitable thickness produced at low current densities have greater tensile strength and exhibit a lesser tendency to warp than sheets of the same thickness produced at higher current densities. Starting sheets produced during a period of about twenty-four hours at a current density of from twenty to thirty amperes per square foot rarely warp during the course of a process. The warping of thin starting sheets takesplace in such a manner that the surface which has been in contact with the aluminum blank during the period of formation of the sheet becomes concave. If a starting sheet exhibits a tendency to warp during the early stages of deposition, it'is advisable to remove it from the electrolytic cell and straighten it before the deposit becomes sufficiently thick to make straightening difiicult or impossible.

We have conducted parallel tests employing metallic zinc starting sheets or cathodes and aluminum starting blanks under different conditions with respect to current density and purity of zinc sulphate solutions obtained from the treatment of ore from the United Verde mine in Arizona, and the results obtained have been consistently better when metallic zinc starting sheets have been employed than when aluminum starting blanks have been employed. In depositing zinc from similar zinc sulphate solutions on zinc starting sheets and on aluminum starting blanks, the current efficiencies in the processes employing zinc starting sheets have invariably excelled the current efiiciencies in the processes employing aluminum starting blanks by from one to five percent.

The excellence of metallic zinc starting sheets as compared with aluminum starting blanks is particularly apparent when the zinc sulphate solutions contain impurities such, for example,

" extending from a point above thesolution line as copper, cobalt and lead. Under such conditions zinc sheets deposited on aluminum blanks are frequently blistered and often contain holes extending entirely therethrough, while zinc deposits on zinc starting sheets usually exhibit no visible defects. We have proven that the aluminum participates in the mischief initiated by the toxic impurities by stripping partly re-dissolved plates of zinc from aluminum blanks and using such plates as starting sheets in the solution from which they were withdrawn. Under such conditions electrodeposition of zinc has been resumed at higher current efficiency and the lead, copper and cobalt holes in the starting sheet have gradually filled with electrodeposited ZlIlC.

Employing a metallic zinc starting sheet produced during the course of twenty-four hours at a current density of about thirty amperes per square foot, we have conducted an electrolytic process for recovering zinc from zinc sulphate solution for a period of three days at a current density of about thirty amperes per square foot with a current efficiency of for the first twenty-four hours, 93% for the second twentyfour hours, and 89% for the third twenty-four hours. The resulting cathode had a thickness seven times the thickness of the usual twentyfour hour deposit on an aluminum blank.

The advantages of zinc starting sheets over aluminum starting blanks in electrolytic processes for recovering metallic zinc from zinc sulphate solutions may be summarized as follows:-

(1) The cost of material for forming the starting blanks is reduced. In the event that electrolytically deposited zinc sheets are employed their production will involve the use of only a relatively small amount of aluminum as compared to the amountrequired by the heretofore customary operations. Rolled .zinc starting sheets may be provided at a lower cost than the cost of aluminum blanks.

(2) The cost of labor and apparatus required for stripping aluminum blanks is eliminated or reduced.

(3) Dross losses are reduced.

(4) Currentemciency is improved, especially when solutions containing small amounts of impurities are employed.

We claim; i

1. In a process for recovering a metal from a solution of a salt of the metal by electrolysis and involving the use of an electrolytic cell having a cathode extending above and below the solution level, the improvement which comprises employing a cathodeprovided with a substantially non-insulating coating of acid resistant material to a point a short distance below the solution line.

2. In a process, for recovering a metal from a solution of a salt of the metal by electrolysis and involving the use of an electrolytic-cell having a cathode extending above and below the solution level, the improvement which comprises employing a cathode provided with a substantially non-insulating coating of rubber extending from a point above the solution line to a point a short distance below the solution line.

3. In a process for recovering metallic zinc from a zinc sulphate solution by electrolysis and involving the use of apartially exposed and partially submerged cathode, the. improvement which comprises employing a cathode comprising a sheet of zinc having its exposed surface and an adjoining portion of its submerged surface provided with a substantially non-insulating 80 coating of acid resistant material.

4. In a process for recovering metallic zinc from a zinc sulphate solution by electrolysis and involving the use of a partially exposed and partially submerged .cathode, the improvement Which comprises employing a cathode comprising an electrolytically deposited sheet of zinc having its exposed surface and an adjoining portion of its submerged surface provided with a substantially non-insulating coating of acid resistant material.

5 In a process for recovering metallic zinc from a zinc sulphate solution by electrolysis and involving the use of a partially exposed and partially submerged cathode, the improvement which comprises employing a cathode having its exposed surface and an adjoining portion of its submerged surface provided with a substantially non-insulating coating of acid resistant organic plastic material.

6. In a process for recovering metallic zinc from a zinc sulphate solution by electrolysis and involving the use of a partially exposed and partially submerged cathode, the improvement which comprises employing a cathode having its exposed sur- 1 5 face and'an adjoining portion of its submerged surface provided with a substantially non-insulating coating of rubber.

7. In a process for recovering metallic zinc from a zinc sulphate solution by electrolysis and involving the use of a partially exposed and partially submerged cathode, the improvement which comprises employing a cathode having its exposed surface and an adjoining portion of its submerged surface provided with a substantially non-insulat- 1 ing coating of mastic. t

8. In a process for recovering metallic zinc from a zinc sulphate solution by electrolysis and involving the use of a partially exposed and partially submerged cathode, the improvement which'com 1 prises employing a cathode having its exposed surface and an adjoining portion of its submerged surface provided with a coating of an acid resistant material of such thickness that zinc ions may pass therethrough during the course of the process.

9. A cathode for mounting above a bath of electrolyte with its lower portion submerged in the electrolyte and its upper portion exposed to the atmosphere above the bath, comprising a metal sheet having its exposed surface and a portion of its submerged surface coated with a layer of organic plastic material of substantially noninsulating thickness.

10. An electrolytic cell comprising a container for electrolyte, and a cathode supported above the 

